1. Field of the Invention
The present invention relates generally to a system and a method for handover in a wireless communication system.
2. Description of the Related Art
To reduce power consumption of a terminal, a wireless communication system supports Discontinuous Reception (DRX). Generally, DRX is classified as idle DRX and connected DRX, according to the state of a Radio Resource Control (RRC) layer.
When the terminal operates in idle DRX, it operates in an RRC standby state. Accordingly, the terminal monitors a Physical Downlink Control CHannel (PDCCH) in a paging interval determined by a base station, and measures a channel to retain mobility. At this time, the network does not include RRC context information of the terminal, and there is no RRC connection between the terminal and the network.
When the terminal operates in connected DRX, it operates in an RRC connection state. Accordingly, the terminal is activated based on configuration parameters for the connected DRX operation provided from the base station using an RRC message. For example, when connected DRX is activated, the terminal monitors the PDCCH during an On Duration time, based on the configuration parameters provided from the base station.
FIG. 1 illustrates conventional connected DRX.
Referring to FIG. 1, when connected DRX is activated, the terminal is activated per DRX cycle 100 to monitor the PDCCH of the base station during the On Duration time 110. The terminal reduces its power consumption in the remaining time 120 in the DRX cycle 100. Basically, the terminal operates in an inactive mode during the remaining time 120.
When the terminal hands over in the wireless communication system, the terminal transmits channel measurement information to the base station. The terminal then hands over to a target base station, based on handover indication information provided from the base station. For example, when the terminal operates in connected DRX, it transmits the channel measurement information to the base station during the On Duration time 110. During the remaining time 120, before the terminal receives the handover indication information from the base station, the terminal cannot receive the handover indication information until an On Duration time 110 of a next DRX cycle 100 arrives. Further, when the DRX cycle 100 is lengthened, the channel state of the serving base station and the target base station changes according to the mobility of the terminal. Consequently, the handover can fail.